Electrolytic polishing solution and the process of polishing therewith



United States Patent 3,365,380 ELECTROLYTIC POLISHING SOLUTION AND THEPROCESS OF POLISHING THEREWITH Yasuichi Shihasaki, Yokohama, Japan,assignor to Kyowa Hakko Kogyo Co., Ltd., Tokyo, Japan, a Japanesecorporation No Drawing. Filed Feb. 24, 1964, Ser. No. 347,061

Claims. (Ci. 204-1405) The present invention relates to an electrolyticpolishing solution and the process of polishing therewith, moreparticularly to the electrolytic polishing solution for effecting apear-like appearance on a surface of aluminum and aluminum alloys andthe usage thereof. The term pear-like appearance used in thisspecification means a surface similar to pear-skin, or uneven, and it isto be construed in a somewhat broader sense including coarsely unevenand finely uneven surfaces with or without luster. The term aluminumalloy means all-kinds of alloys composed of aluminum as a chiefcomponent.

One object of the present invention is to develop a pear-like appearanceon the surface of aluminum and aluminum alloys by electrolyzing.

Another object is to provide a chemical composition of electrolyticsolution useful for the production of pearlike appearance on the surfaceof aluminum and aluminum alloys.

Further object is to provide a novel and economical process forobtaining aluminum and aluminum alloys with pear-like surface byelectrolytic polishing.

In the prior art for polishing an aluminum surface to a pear-likeappearance, mechanical methods such as blasting of sand, Alundum,Carborundum or other particles onto the surface and brushing the surfacewith a turning wire brush have been widely adopted. These prior methods,however, are disadvantageous because they not only are intricate butalso often give unfavorable stresses on the surface of metals.

The present inventor conducted research on electrolytic polishing ofaluminum and aluminum alloys, and has found that when the electrolyticpolishing of aluminum and aluminum alloys is carried out by using anelectrolytic solution containing 3080% of sulfuric acid and a substancehaving the property of adhering to the surface of aluminum and aluminumalloys during electrolysis, the surface of the metal is polished to givepearlike appearance.

Electrolytic polishing by using sulfuric acid has already been known,but such polishing has been conducted to polish the metal surface togive mirror like appearance, and the application of such electrolyticpolishing to pearlike appearance has never been reported.

As above-mentioned, the present inventor has found a novel process forapplying electrolytic polishing to produce pear-like appearance.

The electrolytic solution in the present invention is a solutioncontaining 30-80% of sulfuric acid and'a substance having the propertyof adhering to the surface of metal during electrolysis, and theelectrolysis is carried out by using aluminum and aluminum alloys asanode. In the electrolysis of the present invention, the adheringsubstance adheres nonuniformly to the surface of the metal duringelectrolysis or maintains small bubbles of generated gas or oxygen atsuch surface, thus the surface of metal is dissolved nonuniformly andgives a pear-like appearance. This is the most important principal ofthe present invention. As to the concentration of sulfuric acid,

30-80% may be used and 5070% is most favorable.

When the concentration of sulfuric acid is lower, the surface of metalis covered with oxide film and does not give a good pear-likeappearance.

3,305,380 Patented Jan. 23, 1968 As a result of studies, it was foundthat good pear-like appearance is not obtainable at a sulfuric acidconcentration lower than 25%, and inversely at a concentration higherthan 80% good pear-like appearance can be obtained technically but fromthe industrial viewpoint this condition has little practicabilitybecause the high viscosity of the sulfuric acid decreases the efiiciencyof polishing work. Consequently, the concentration of sulfuric acid issuitable to be in the range of from 30% to 80% by weight and theconcentration of from 50% to is recommended as an optimum range forpractical purposes.

In some cases, a part of the sulfuric acid may be replaced by phosphoricacid, although it is necessary to keep the phosphoric acid concentrationlower than 30%. The existence of sulfate, such as sodium sulfate andaluminum sulfate, in electrolytic solution is not injurious todeveloping of a good pear-like appearance, if the concentration ofsulfate is not of a high value. As to the adhering substances, thesubstances added to the sulfuric acid solution are effective bythemselves as the adhering substances but sometimes the added substancesare converted in sulfuric acid and become effective.

As to the added substances which give the adhering effect, many kinds ofmaterials may be used both singly and conjointly, and the following areexamples: gluconic acid; gluconates; natural high molecular weightsubstances such as glue and agar; carbohydrates such as starch,cellulose, glucose, fructose, sucrose and maltose; tannic acid; higherfatty acids; higher fatty acid derivatives, saccharin; polyhydricalcohols such as glycerine; synthetic high molecular weight substancessuch as polyethylene and polyvinyl alcohol. The substancesabovementioned are generally effective as the adhering substance, butsome substances change in the sulfuric acid and the changed substancesare also effective; moreover, in some cases, the changed substances aremore effective than the originals. Furthermore, substances such aspolyethylene become effective when changed. When gluconic acid orgluconate is used as the additive substance, the total concentration isnot critical and even a low concentration of 0.05% to 10% givessatisfactory results. Higher concentrations of gluconic acid areacceptable technically but inconvenient to practical operation becauseof the high viscosity of the solution. When other substances thangluconic acid and gluconate are used as additives, the concentration isalso not critical but 0.01-10% is favorable in practical operation.

As to the electrolyzing conditions, the temperature of the electrolyteis not critical, however, 3090 C. may be suitable, the optimum range is60-80 C., and 525 volts is the most formable bath voltage. Asbefore-mentioned, aluminum and aluminum alloys are the anode, and as thecathode, many kinds of electrodes may be used. Carbon and lead are themost preferable cathodes.

The anodic current density may vary depending upon the composition andtemperature of the electrolytic solution but usually it shows maximumamperage at the starting period of electrolysis and then diminishes withlapse of time. At about one minute after initiation of the electrolysisthe current becomes two-thirds to one-fifth of the initial value andthereafter approaches an approximately constant value asymptotically.When the current is 30 a.dm.- to 50 a.dm.- at first and 5 a.dm.- to 15a.dm.- one minute later, a good pear-like appearance is attainable on analuminum surface. If the aluminum anode is vibrated, the current densityincreases unstably and electrolyzing sometimes exhibits a tendency togive the disappearance of the pear-like appearance.

The time required for the electrolysis is two to ten minutes normallybut it should more definitely be determined with due consideration foruniformity and unevenness of pear-like surface.

The conditions of pear-like appearance obtained on the finishing surfacevary in practice according to the composition of the electrolyticsolution, liquid temperature, purity of aluminum, composition ofaluminum alloy and other conditions. In order to produce a uniform andline surface of pear-like appearance the several conditionsabove-mentioned must be kept within the optimum range in considerationof the correlation between them. Pearlike appearance on aluminum metalsurface developed in accordance with the present invention is fine andbeautiful. Compared with mirror-surface polishing, moreover, pear-likeappearance by electropolishing offers an advantage in that small scarson the aluminum surface are not distinguished and makes short orcompletely unnecessary the buffing of the aluminum surface prior to theelectrolysis.

It falls Within the scope of knowledge of engineers concerned thatpost-treatment such as surface oxidizing, dyeing and sealing can beapplied on the aluminum surface of pear-like appearance. Moreover, inthe present process, the pear-like surface is often anodized and is dyedwith dyestuif solutions; therefore, the anodic oxidation may be omitted.

The following examples show the nature of the present invention ingreater detail. They are only for the understanding but not for thelimitation of the present invention.

Example 1 A solution consisting of 300 kgs. of sulfuric acid, 200 kgs.of water and 2 kgs. of gluconic acid is used as the electrolyte(electrolytic solution). An aluminum plate having 2 mm. thickness and 30dni. surface area is used as anode, and lead electrode is used ascathode. The electrolysis is carried out at 60 C., the bath voltage is9-12 volts, and the current density is 60-20 a.dm.- After 3 minutes, thealuminum plate is taken out and washed with water. The surface of theplate shows fine pear-like ap- What I claim is:

1. A method for electrolytically polishing a metal selected from thegroup consisting of aluminum and alum-inuni alloys to effect a pear-likeappearance thereon which comprises electrolyzing a solution consistingessentially of Water, from 30 to 80% by weight of sulfuric acid and aneffective amount of an additament substance selected from the groupconsisting of gluconic acid, alkali metal gluconates, glue, agar,starch, cellulose, glucose, fructose, sucrose, maltose, polyethylene,polyvinyl alcohol, tannic acid, stearic acid, stearates, benzoates,glycerin and saccharin at a temperature of from 30 to 90 C. and avoltage of from 5 to volts, the metal to be polished constituting theanode, until the desired pear-like appearance is attained.

2. The method of claim 1, wherein the current density during theelectrolysis is from 5 to 70 amperes per square decimeter.

3. The method of claim 1, wherein said solution also contains up to byweight of phosphoric acid.

4. The method of claim 1 wherein said additanient substance is a memberselected from the group consisting of gluconic acid and alkali metalgluconates.

5. The method of claim 1, wherein said additament substance is a memberselected from the group consisting of glue and agar.

6. The method of claim 1, wherein said additament substance is a memberselected from the group consisting of starch, cellulose, glucose,fructose, sucrose and maltose.

7. The method of claim 1, wherein said additament substance is a memberselected from the group consisting of polyethylene and polyvinylalcohol.

8. The method of claim 1, wherein said additament substance is a memberselected from the group consisting of tannic acid, stcaric acid,stearates and benzoates.

9. The method of claim 1, wherein said additament substance is glycerin.

pearance with luster.

Example 2 Run Composition of Electrolytic solution Temp. Voltage CurrentTime Material to be No. (ratio by weight) C.) (v.) (D insity (min.)polished Results 1 Sulfuric.3 acid, 56; Water, Gluconic 70 9-12 54-18 399.9% A1 Lustrous pear appearance.

Duralumin.. Grey-colored, coarse, slightly uneven pear appearance. 2Phosphoric acid, 24; Sulfuric acid, 52; 72 8 67-18 3 99.5% AlSemi-lustrous, some-what non- Water, 90; Sodium gluconate, 10. uniformpeer appearance. 3 Sulfuric acid, 85; Water, 45; Gluconic 45 8 20-30 1099.7% Al Semi-lustrous, non-uniform acid, 2. slightly uneven pearappearonce.

4 Sulfuric acid, Water, 69; Gluconic 62 7. 5 40-24 3 99.9% Al Lustrous,somewhat nonacid, 20. uniform pear appearance.

5 Sulfuric acid, 60; Water, 40; Glycerine, 70-80 10-12 35-9 3 99.7% A1Lustrous, good, fine, uniform 5-10. pear appearance.

6 Sulfuric acid, Water, 45; Z-ethylhex- 70 10 55-19 3 99.7% Al Lustrous,good, uniform pear ylammonium stearate, 0.5. appearance.

7 Sulfuric acid, Water, 40; Saccharin, 70-75 11 50-8 3 99.7% AlLustrous, good, finely shallow,

1.0. h uniform pear appearance.

8 511611511110 acid, 60; Water, 40; Stearic acid, 70 9 50-10 4 99.7% AlLustrous, good, uniform pear a carancc.

9 SuIlfuric acid, 60; Water, 40; Tannic acid, -75 10 45-6 4 99.7% Al 3 130.

10 Sulfuric acid, 55; Water, 45; Agar, 0.5.-.. 60-75 12 -12 3-4 99.7% A1Semi-lustrous, good, slightly coarse, uniform pear appearonce.

11 Sulfuric acid, 55 ;Water, 45; Glue, 0.5 72 11-12 56-21 3 09.7% AlLustrous, good, slightly coarse,

uniform pear appearance. 12 Sulfuric acid, 55; Water, 45; 2-ethylhex- 7010 40-20 3 Duralumin.. Semi-lustrous, shallow, coarse, ylainmoniumbenzoate, 0.5. somewhat non-uniform pear appearance. 13 Sulfuric acid,55; Water, 45; Sawdust 11 50-20 3 99.7% Al Lustrous, good, uniform pear(cellulose), 1. appearance. 14 Sulfuric acid, 60;Water, 40; Polyethylene65-70 10 55-10 3 99.7% Al Lustrous, good, fine, uniform (but slightlysoluble), 0.1. pear appearance.

15 Sulfuric acid, 55; Water, 45; Polyvinyl 70-75 5-10 40-10 3 99.9%AL..- Lustrous, good, uniform pear alcohol, 0.15. appearance.

16 ..do 70 5-10 40-10 3 Duralumin.... Grey-colored, slightly uneven pearappearance.

10. The method of claim 1, wherein said additament substance issaccharin.

References Cited UNITED STATES PATENTS Chester 204145 Lagut 260-301Easley 252-79.4 Hesch 204-140.5 McDonald 252-794 Turner 204140.5

6 9/1956 Easley et a1 252-79.4 1/1961 Cybriwsky et a1. 204140.5 10/1963Parkinson 25279.4 11/1964 Neunzig 204140.5

FOREIGN PATENTS 7/1952 Great Britain.

ROBERT K. MIHALEK, Primary Examiner. JOHN H. MACK, Examiner.

1. A METHOD FOR ELECTROLYTICALLY POLISHING A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS TO EFFECT A PEAR-LIKE APPEARANCE THEREON WHICH COMPRISES ELECTROLYZING A SOLUTION CONSISTING ESSENTIALLY OF WATER, FROM 30 TO 80% BY WEIGHT OF SULFURIC ACID AND AN EFFECTIVE AMOUNT OF AN ADDITAMENT SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF GLUCONIC ACID, ALKALI METAL GLUCONATES, GLUE, AGAR, STARCH, CELLULOSE, GLUCOSE, FRUCTOSE, SUCROSE, MALTOSE, POLYETHYLENE, POLYVINYL ALCOHOL, TANNIC ACID, STEARIC ACID, STEARATES, BENZOATES, GLYCERIN AND SACCHARIN AT A TEMPERATURE OF FROM 30* TO 90*C. AND A VOLTAGE OF FROM 5 TO 25 VOLTS, THE METAL TO BE POLISHED CONSTITUTING THE ANODE, UNTIL THE DESIRED PEAR-LIKE APPEARANCE IS ATTAINED. 